well this is a very general question.... but there are things you can and cant have together in your tank...depending on tank size.
for example, you can not have a saddleback butterfly fish with any anemones, for the butterfly fish will eat anemones.
triggerfish should not be put in with a reef tank smaller than 100 gallons... he will eat almost any coral you put in there with him.
2 different kinds of clownfish can not be in a tank less than 30 gallons... its just too small and clownfish are too territorial.
Things that go well together would be like a bubble tip anemone, and a true percula clown.
anyways if you want a more thorough answer, try asking a little more specific of what you want to know |